Linear charger circuits are widely used for portable IoT devices. Wearable devices like fitness bands and watches require chargers with widely adjustable charge current capability e.g. 5 mA up to 500 mA. In this charge current region the accuracy of the charging current as well as the termination current accuracy play a significant role. High accuracy allows delivering fast charge rates to the IoT device. FIG. 2 shows a typical battery current IBAT to voltage VBAT relationship 310 of a linear charger circuit. Typically, a charge operation has two phases, a first charging phase 330 and a second charging phase 320. During the first charging phase 330, charge current is supplied to an electric load (e.g., battery) at a constant charge current value ICHARGE under control of a constant current (CC) loop. The first charging phase 330 may also be referred to as CC charging phase. As soon as a certain (target) voltage value is reached by the voltage VBAT of the electric load, charging continues, in the second charging phase 320, but now the charge current is supplied at constant voltage under control of a constant voltage (CV) loop. The second charging phase 320 may also be referred to as CV charging phase. The charge operation terminates once the actual charge current IBAT reaches the value of the termination current ITERM.
One of the main specification parameters for the linear charger circuit is the accuracy of the constant charge current in the constant current loop, ICHARGE. The second important linear charger circuit specification parameter is the accuracy of the termination current ITERM, i.e., the current at which the charge operation is actually terminated.
The accuracy of the charge current ICHARGE and the termination current ITERM is in both cases defined by the constant current CC loop structure, as both of them depend on a sense ratio (in terms of device sizes) between a pass device and a sense device of the linear charger circuit.
For high charge currents ranges (e.g., ICHARGE>200 mA) the accuracy of the CC loop typically is sufficient and single digit accuracy is typically achievable for the charge current. However, for lower charge current ranges such as for charge currents that occur in the second charging phase 320 in FIG. 2, the accuracy of the CC loop significantly degrades, due to the comparatively large sense ratio between the pass device and the sense device (in terms of device sizes). Accordingly, termination of the charge operation at the intended termination current cannot be guaranteed. On the other hand, reducing the sense ratio would result in unnecessarily high current consumption, and hence lower power efficiency, for high charge currents.